The role of sea-salt particles as cloud condensation nuclei over the remote oceans

If recent measurements of the fluxes of sea-salt particles over the oceans as a function of wind speed are extrapolated to include the smallest partic les detected over the oceans and in laboratory experiments on bursting bubb les, the flux F of particles having radii between r(0) = 0. 1 mum and r mum at 80% relative humidity may be represented by F = 2U(3.5)(e(-0.2r0) - e(- 0.2r)) m(-2)s(-1), where U is the near-surface wind speed in in s(-1). The flux of particles of r less than or equal to 5 mum in winds of 16 in s(-1) is 20 000 m(-2)s(-1). The corresponding local rate of production by air bub bles bursting in the foam patches of breaking waves that occupy, on average , only 5% of the ocean surface at this wind speed is 40 cm(-2)s(-1). The av erage concentration of particles over the sea area producing whitecaps is: N = integral (r)(r0) (1)/(vr) (dF)/(dr) dr = 0.4U(3.5) integral (5 mum)(r0= 0.1 mum) (exp(-0.2r) dr)/(vr), where v(r) is the deposition velocity for particles ofradius r. In winds of 16 in s(-1) the total concentration of sea-salt particles is calculated to be 40 cm(-3). These, and the fluxes and concentrations calculated for othe r wind speeds, are consistent with measurements made at sea and in the labo ratory by many scientists during the last 50 years. Recently O'Dowd et al. have reported on sea-salt particles of dry radius as small as 0.03 mum, with radius 0.06 mum at 80% relative humidity and mass 5 x 10(-16) g. If the above equation is extended to include such small part icles, it predicts a total concentration of 60 cm(-3) in winds of 17 in s(- 1), fairly close to the reported 70 cm(-3). In general these computed and observed concentrations of sea-salt particles are adequate to account for observed droplet concentrations in clouds over remote oceans uncontaminated by continental aerosols. However, on rather r are occasions droplet concentrations in maritime cumulus exceed 100 cm(-3). Observations are cited to suggest that, in such cases, sea-salt nuclei may be augmented by biogenic particles originating at the sea surface and by s ulphate/sulphuric acid nuclei produced by the absorption of derivatives of dimethyl sulphide in cloud droplets. Locations such as Hawaii and Tasmania are occasionally contaminated by cont inental or local pollution which may account for the rare reports of drople t populations being as high as 400 cm(-3).